api: round-trip drops accidental editorial attributes (cautionary/editorial/parentheses)

[webern]

Surfaced by the reworked round-trip classifier (PR #225; part of #212).

What

<accidental> loses its editorial attributes on write.

Verified on lysuite/ly01e_Pitches_ParenthesizedAccidentals.xml:

• expected: <accidental editorial="yes">
• actual: <accidental>

data/api.features.xml already marks accidental cautionary/editorial/parentheses as partial — they read but do not round-trip. parentheses is likely the same code path and should be fixed together.

Impact (reorder-free candidate files; 550 total)

• attr:accidental@editorial 5, attr:accidental@cautionary 4

Repro

make dump-api-roundtrip && make classify-api-roundtrip
# build/api/classified.json: signatures attr:accidental@editorial / attr:accidental@cautionary

References

• Part of api: expand corpus coverage of the api round-trip pass-list #208; feeds api: implement ranked fixes and grow the round-trip corpus baseline #213
• Surfaced by PR audit: classify api round-trip failures by measured divergence and rank a worklist #225